The invention relates generally to vacuum cooling processes and more specifically to a method of and apparatus for vacuum cooling processing of produce and similar foodstuffs.
The vacuum cooling of foodstuffs is a technique developed approximately forty years ago wherein produce, particularly such succulent items as lettuce are subjected to a vacuum which effects vaporization of a certain portion of the water contained therein and thus cooling of the produce. Much early development in this technological area was done by Morris Kasser and is related in U.S. Pat. Nos. 2,344,151 and 2,651,184. The former patent teaches a vacuum chamber wherein pressurized steam is utilized to produce a vacuum as well as a thermostatic controller which senses dry bulb temperature and terminates the vacuum cooling cycle when the temperature of the chamber reaches a preset level. Since the time of this patent, it has become known that sensing dry bulb temperature is not an accurate manner in which to determine the actual temperature of the produce since such temperature is affected by water evaporation which, of course, does not take place with and is not sensed by a conventional dry bulb thermometer or a thermostat.
This shortcoming was realized and a solution thereto is disclosed in U.S. Pat. No. 2,585,086. Here, a thermometer bulb is surrounded by a wick which is kept saturated throughout the cooling process. The thermometer thus senses an approximation of the temperature of the product as moisture is vaporized therefrom. It is apparent, however, that the characteristics of a saturated wick wet bulb thermometer will vary from those conditions and temperatures exhibited by actual produce and that such a temperature sensing method will provide much improved performance over a dry bulb temperature sensing scheme though it, too, is not wholly accurate.
Other apparatus and methods have been developed to monitor the ongoing cooling process as a means of determining the appropriate termination point. One such approach is disclosed in U.S. Pat. No. 4,204,408. In this disclosure, dimensional change resulting from the vaporization of water from foodstuffs is sensed and utilized to determine an appropriate cooling rate.
The foregoing discussion suggests that if the temperature of produce or foodstuffs could be accurately determined during an evaporation cycle, determination of the appropriate termination point of the cooling cycle could be achieved. This is not completely true. First of all, the temperature of the produce can vary from location to location within the cooling chamber. Secondly, and more importantly, is the temperature differential existing between the outside and the inside of a particular foodstuff such as head lettuce. Typically, the outer surface of lettuce will cool somewhat more rapidly than the inner surfaces and interior and thus exhibit a somewhat lower temperature than the inner surfaces during the greater part of the cooling cycle. Finally, differences in the evaporative cooling rates of various produce, will affect the cycle time and to a lesser extent, the temperature uniformity of the produce. Such differences are the result of both the mass to surface ratio of the produce and the ability of that surface to give up moisture. For example, since head lettuce releases its water relatively rapidly, cooling to achieve a given temperature can be performed somewhat more rapidly than on larger, more dense vegetables such as carrots which may have their exterior surfaces cooled to a desired temperature relatively rapidly but which may cool internally much more slowly and thus exhibit both significant temperature gradients as the cooling process nears an end and may further warm significantly after the process has been terminated.
It should also be noted that most produce is exceptionally sensitive to even brief freezing and that significant precautions must be made in order to ensure that under no circumstances does the vacuum and corresponding wet bulb (evaporative) temperature within the vacuum chamber drop to a point that freezing occurs.
The foregoing properly suggests that the control of vacuum cooling processes requires careful and skilled operator attention in order to properly control the duration of the vacuum cooling cycle, the maximum vacuum and the minimum temperature achieved. One approach to ensuring safe cooling cycles is to perform what may be considered partial cycles, read the temperature of such produce by various temperature sensing units disposed within the produce itself, after having allowed the temperature to stabilize and repeating partial cycles at increasingly deeper vacuums until such desired minimum temperature is achieved. Such an approach can extend the per load cycle time from an average of between 15 and 20 minutes to twice these times or more. Such an approach has an obvious deleterious effect on the capacity of a given vacuum chamber and thus may necessitate the purchase of additional units, ultimately resulting in an increase in operating costs.
From the foregoing, it can be appreciated that minimum cycle times are desirable since they will maximize capacity of a given vacuum chamber. But is is also apparent that such minimum cycle times can be achieved only through careful control of the vacuum process unless uneven cooling or freezing of produce and foodstuff damage is to occur. It is apparent, therefore, that improvements in the art of vacuum cooling methods and apparatus are both possible and desirable.